Technology Overview: Webb's Nuts & Bolts

The James Webb Space Telescope
is an infrared-detecting observatory
that will soar through space at the
Second Sun-Earth Lagrange Point,
an orbit far beyond Earth's Moon.
Webb's giant sunshield will protect
it from stray heat and light, while its
large mirror enables it to effectively capture infrared light, bringing us the clearest picture ever of objects that emit this invisible radiation — early galaxies, just-forming stars, clouds of gas and dust, and much more.

Webb has an ambitious design that tackles the two main challenges for an infrared telescope: it has to have a large mirror, in order to best capture the long infrared wavelength; and it has to be kept cold, in order to keep unwanted sources of infrared from interfering with the emissions it attempts to detect.

WEBB'S ORBIT

Lagrange points, named after their discoverer, Joseph Louis Lagrange, are five special points around two orbiting bodies where gravity allows a third, smaller body to orbit at a fixed distance from the larger bodies. In our case, these two bodies are the Sun and the Earth. The Webb telescope is being sent to one of these stable orbital points via rocket.

Webb's planners have several good reasons for this choice. At the Second Lagrange Point L2), 940,000 miles (1.5 million km) from Earth, the telescope will be able to keep its tennis court-sized sunshield between its sensitive equipment and the Earth, Sun and Moon, yet remain in an orbit that makes operations and communications easy. Just as important, Earth won't obstruct the telescope's view.

CLIMATE CONTROL

Although an L2 orbit offers several advantages, the most important is its location out beyond Earth. Warm objects emit infrared light in great amounts. That means the infrared detectors in Webb's instruments must operate at very cold temperatures (about -375 degrees Fahrenheit, or 40 Kelvin or -233.3 degrees Celsius). Without cooling, these instruments wouldn't be able to see beyond the radiation they generate on their own. Webb would be unable to carry enough of its own coolant and fit inside its rocket, so a cold orbit around the Sun offers an ideal solution.

RUGGED DESIGN

At this distant orbit, the Webb telescope is too far from Earth to have the protection of our planet's magnetic field, which blocks high-energy cosmic rays. Cosmic rays can interfere with the telescope's signals or even build up electrical charges that can create the equivalent of small lightning strikes on the telescope. Such sparks can hurt sensitive equipment or damage the telescope's materials. Webb has been engineered to take this into account, with extra shielding for detectors and conduction areas in the sunshield to prevent voltages from accumulating.

Webb doesn't look like your typical telescope. This is because it's not enclosed in a tube or dome. Telescopes that see primarily visible light, like Hubble, use tubes to keep stray light from entering their instruments. But Webb's infrared detectors have to be protected primarily from heat sources, so it uses an open design that allows heat to dissipate easily into space. Webb's sunshield, which unfurls after Webb is released from its rocket, consists of five layers of a heat-resistant material called silicon-coated Kapton. Each layer further deflects any heat or light that penetrates the previous layers.

WEBB'S MIRROR

Webb's most innovative component is its primary mirror, which will be folded up to fit inside the rocket that launches it into space, and unfold once it nears its orbit.

The mirror consists of 18 extremely lightweight but rigid segments. The segments are made of beryllium, a material capable of handling the extreme cold the Webb telescope will face: -233.3 Celsius (40 Kelvin, -388 degrees Fahrenheit). Beryllium contracts and deforms less than glass at this cold temperature, and thus remains more uniform.

The Webb mirror segments will also be coated with a fine layer of 24-karat gold. This gold coating is used in infrared telescopes because it reflects red light extremely well. Using gold will make the mirror 98 percent reflective; ordinary mirrors are only 85 percent reflective.

I SPY

Once Webb reaches its orbit, operators will go through a complex focusing procedure to bring all its segments into alignment, allowing them to operate as a single, unified mirror. At 21.3 feet (6.5 m), Webb's mirror will have about seven times the area of Hubble's and 50 times the area of the Spitzer Space Telescope, NASA's current orbiting infrared observatory. The resolution of the new observatory will be three times more powerful than Hubble in the infrared and eight times more powerful than Spitzer.

Hubble can see to about 800 million years after the Big Bang, while Webb will be able to see to about 200 million years after the Big Bang. It may not sound like big difference, but like any newborn, the universe changed drastically and quickly in its early life. Looking at the early universe at different stages is like the difference between viewing pictures of a baby and pictures of a teen. The infant pictures will change swiftly, within days, while the teen pictures will hardly differ at all from week to week.

WEBB'S INSTRUMENTS

Infrared wavelengths are a bit longer than the optical wavelengths of the spectrum that are visible to humans. Although we cannot see infrared with the naked eye, we can sense it as heat. Infrared wavelengths are broken down into near, mid, and far infrared.

Engineers, scientists, and the average person make use of infrared technology in everyday life: in security systems, television remote controls, and in probes for remote diagnostics in industry, science, and the arts.

Webb will carry four instruments that are sensitive to the infrared wavelength bands of the electromagnetic spectrum.

Near-Infrared Camera (NIRCam) — NIRCam will look at near-infrared wavelengths. It will detect light from the earliest stars and galaxies in the process of formation, stars in nearby galaxies, and Kuiper Belt objects and young stars in the Milky Way.

Near-Infrared Spectrometer (NIRSpec) — NIRSpec is called a “multi-object near-infrared spectrometer.” A spectrometer is a device that breaks light up into its colors. NIRSpec will see near-infrared wavelengths and will be capable of observing more than 100 objects simultaneously. It is designed to study, among other things, star formation and the chemical composition of young, distant galaxies.

Mid-Infrared Instrument (MIRI) — MIRI will study mid-infrared wavelengths, examining distant stellar populations, the physics of newly forming stars, and the sizes of faintly visible comets and objects from the region of our solar system called the Kuiper Belt.